Your child knows the rule in A-level Biology: water moves from high water potential to low. But then a question gives two values like minus 200 and minus 500, and they draw the arrow the wrong way. Sometimes the difficulty is not the osmosis rule itself, but ordering the negative values correctly.
The quick version
- The rule is simple: water moves from higher water potential to lower.
- The minus signs can be one source of the confusion.
- A value like minus 200 is higher than minus 500, even though 500 looks bigger.
- There is a short number-line task at the end you can try at home this week.
Where water potential questions go wrong
A child who gets this wrong can still be good at Biology. They may know the higher-to-lower rule but still misread the negative values. The number-line step is one place to check. Water potential is written as a negative, like minus 200 kilopascals. A value like minus 200 is higher than minus 500, even though 500 looks bigger. So the arrow gets drawn backwards. Separating the number-line step from the Biology makes it easier to see which part needs work.
The water potential number line
For these simple solution questions, a number line sorts out the minus signs. Water potential is measured in kilopascals, written kPa.
Water moves from higher water potential to lower water potential.
0 kPa is the highest. Adding solute to water lowers its water potential, so solutions sit below zero. A smaller negative number, like minus 200, is higher than a larger negative number, like minus 500.
Minus 200 kPa is higher than minus 500 kPa. So water moves from the minus 200 solution to the minus 500 solution.
This number line is a tool for these simple solution comparisons. It is not the whole story for every situation in a plant.
How we teach the direction
We split the job in two. First the number line, then the Biology. We get the child to say which value is higher before they draw a single arrow. A smaller negative number is higher. Once the ordering is clear, the rule does the rest: water moves from higher water potential to lower. It is a Greenhill way of separating the sign/number-line step from the Biology, so each can be checked on its own.
Greenhill tutor
Kriszta, Biology tutor
Kriszta studied Biological Sciences at the University of Oxford with a First, and teaches A-level Biology with Greenhill.
Arrow going the wrong way?
Tell us where your child’s osmosis answers break down. A tutor can look at one and see if it is the numbers or the Biology.
Ask about a tutorTwo solutions, one arrow
These numbers are invented to show the direction. They are not from a real pupil’s paper.
The question
Solution A has a water potential of minus 150 kPa. Solution B has a water potential of minus 400 kPa. Which way does the water move?
Reading the signs
Minus 150 is a smaller negative number than minus 400, so solution A has the higher water potential. Water moves from higher to lower, so it moves from solution A into solution B. The minus signs make it look backwards, but the rule holds.
Try this at home this week
This takes about eight minutes. It gives you a quick way to see whether ordering the negative values is one part your child needs to practise.
What you need. Paper, a pen, and the pairs of values below, which we have written out for you.
The question. Here are four pairs of water potential values, in kPa. For each pair, your child draws a small number line, marks both values, and draws an arrow for which way water moves. The pairs are: minus 200 and minus 500; minus 700 and minus 300; 0 and minus 400; and minus 100 and minus 600.
- For each pair, draw a small number line with 0 at the top end.
- Mark both values on it.
- Circle the higher one. Remember, a smaller negative number is higher.
- Draw an arrow from the higher value to the lower value. That is the way water moves.
- Write one sentence saying which way the water moves and why.
What to watch. Watch which part is the problem. If your child orders the negative numbers wrongly on several pairs, the sign/number-line step is worth practising separately. This short task does not rule out a Biology misunderstanding as well.
What it cannot tell you. This short task does not tell you whether your child can handle pressure potential, longer plant-water questions, or working from experimental osmosis data.
Where a tutor takes it next. That is where a tutor picks it up: first sorting the number line from the Biology, then building up to full A-level questions.
This clip shows a maths lesson recorded in Pencil Spaces, the online classroom used by Greenhill Academics.
Our A-level Biology tutors
Kriszta studied Biological Sciences at the University of Oxford, graduating with a First and ranking in the top ten of her year. She teaches Biology with Greenhill.
Jessica is a fourth-year Medicine student at the University of Cambridge who achieved A*A* in Biology and Chemistry at A level. She teaches Biology and Chemistry with Greenhill.
Questions parents ask us
My child keeps getting the direction of water movement wrong. Why?
The Biology rule is simple: water moves from higher water potential to lower. One possible difficulty is the minus signs. A value like minus 200 is higher than minus 500, which looks backwards, so the arrow gets reversed.
What is the rule you teach?
Water moves from higher water potential to lower water potential. On the number line, 0 kPa is the highest, and adding solute makes the value more negative, so solutions sit below zero.
Is this an A-level topic?
This article is written for A-level Biology. Water potential and osmosis come up across cells, transport and plant questions, so the higher-to-lower rule is used when deciding the direction of water movement in these questions.
Do you tutor A-level Biology online?
Yes. We tutor A-level Biology online for UK and international families. Tell us the exam board and where your child gets stuck, and we can talk through the help they need.
How quickly can this improve?
It depends on the child and the exam board. The rule is short, but using it reliably takes practice on real questions. A tutor can show you what that practice looks like.
Want a tutor to work on A-level Biology with your child?
Tell us the exam board and where your child gets stuck, and we can talk through the help they need.
Send an enquiry